In chromatographic apparatus and procedures, closures for containers, columns and the like are commonly in the form of appropriately shaped bodies of silicone rubber or similar material which can be penetrated by a hypodermic needle for access to the contents of the container with immediate healing of the hole pierced in the body by the needle when the needle is withdrawn to maintain the sealing closure. The escape of any part of the contents of the container and access of the atmosphere are both prevented by the use of such septa.
While very convenient, the use of such septa is not without some problems. A major problem results from the bleeding from the silicone rubber due to the action of the material confined in the container or chromatographic system, resulting in contamination of the material. The problems are aggravated at higher temperatures. Attempts to isolate the silicone rubber from access by the contents of the containers have been partially successful in mitigating the contamination problem. The approach has been to provide a barrier coating, most commonly tetrafluoroethylene, upon at least the surface of the septum body which is to be exposed to the interior of the container or chromatographic system. Teflon offers only limited stability at high temperatures and contains fluorine, a halogen, which in even trace quantities greatly interferes with radio-active detectors, such as the highly sensitive electron capture detectors used for trace pesticide or organic halide analyses.
The object of this invention is to provide chromatographic septa which may be safely employed to close containers, columns, and the like, especially in chromatographic apparatus, which are provided with barrier coating that is inert to substantially all known organic solvents and is stable to about 400.degree. C. Septa bleed is effectively minimized and the barrier coating does not affect electron capture detectors.
This object is attained by providing silicone rubber septa having a coating of polyimide polymer applied directly upon and securely bonded to the surface of the septa. If desired for certain applications, the polymide coating may itself be coated with Teflon to enhance resistance of the barrier coating to moisture.